Wave generator



G. E. WEIBEL WAVE GENERATOR Feb. 16, 1960 Filed Feb. 12, 1957 2Sheets-Sheet l F- 0 P m a CW n a x Fig-2 TIME ' INVENTOR GER/MRO E.WE/BEL BY 34 ATTORNEY Feb.- 16, 1960 s. E. WEIBEL 2,925,523

WAVE GENERATOR Filed Feb. 12, 1957 I 2 Sheets-Sheet 2 LOGAR/THM/C'FUNCTION ELECTRON FUNCTION BEAM INVENTOR GER/MRO E. WE/EEL ATTORNEYUnited States Patent WAVE GENERATOR Gerhard E. Weibel, Manhasset, N.Y.,assignor, by mesne assignments, to Sylvania Electric Products Inc.,Wilmington, Del., a corporation of Delaware My invention is directedtoward wave generators.

In the electronic arts it has become necessary to generate, atrelatively high power levels, electromagnetic waves at wavelengthsshorter than a centimeter; i.e. millimeter and submillimeter waves. Ihave invented a device (which I define as a wave generator) that can beused for this purpose.

Accordingly, it is an object of my invention to generate millimeter andsubmillimeter waves at relatively high power levels.

Another object is to provide new methods for generating electromagneticwaves.

Still another object is to provide new types of devices for generatingelectromagnetic waves.

Yet another object is to provide new millimeter and submillimeter wavegenerators which can generate millimeter and submillimeter waves atrelatively high power levels.

A further object is to accelerate electrons in such manner thatsubmillimeter waves are radiated therefrom.

Another object is to compress a large number of electrons into a verysmall volume element, comparable or smaller than the wavelength of theradiation to be generated.

Still a further object is to provide new millimeter and submillimeterwave generators in which are incorporated means for acceleratingelectrons in such a manner that millimeter and submillimeter waves areradiated from these electrons.

These and other objects of my invention will either be explained or willbecome apparent hereinafter.

As is well known to the art, an electron, when accelerated, will radiateenergy in the form of electromagnetic waves, the radiated powerincreasing as the magnitude of the acceleration increases.

In accordance with the principles of my invention, I exert a combinationof electric and magnetic forces upon an electron bunch containing one ormore electrons in such manner as to cause the bunch to traverse acycloidal path. Under these conditions, the electrons in the bunch willbe so accelerated as to radiate electromagnetic waves of extremely shortwavelength, as for example, millimeter or submillimeter waves.

More particularly, I confine the electron bunch within a radiationchamber. Two quasi-stationary fields, a uniform electric field and auniform magnetic field, are established within the chamber, the fielddirections being such that the electric field vector is alwaysperpendicular to the magnetic field vector. The electric and magneticforces produced by these two fields act upon the bunch in such mannerthat the bunch traverses a cycloidal path and the electrons in the bunchradiate in the manner indicated previously.

In one application of my invention the cycloidal path traversed by thebunch is such that the cusps of the path define a straight line which isperpendicular to the magnetic field vector and which intersects theelectric field vector at an acute angle. This type of path is obtainedwhen the direction of the electric field does not change; the electricfield vector is fixed in position.

In a second application, the path traversed by the bunch is such thatits cusps define a circle which lies in a plane perpendicular to themagnetic field vector or stated difierently, the bunch describes anepicycloid. This type of path is obtained when the quasi-stationaryelectric field is circularly polarized in a plane perpendicular to themagnetic field vector; i.e. the electric field vector of constantmagnitude rotates uniformly in a plane perpendicular to the magneticfield vector.

In all cases, the power level of the radiated waves increases as thenumber of electrons in the bunch increases and the dimensions of thebunch are reduced to the order of or less than the wavelength of theradiation to be generated; and hence, relatively large amounts of powercan be obtained by use of a tightly compressed bunch containing a largenumber of electrons. It is to be understood that the term bunch as usedherein refers to a collection of one or more electrons which in so faras the generation of radiation is concerned, can be consideredequivalent to a single particle carrying an electric charge and alsohaving a mass equal to the sum of the masses of all electrons in thebunch. Stated difierently, all of the electrons in the bunch move withthe same phase and act as one rigid body.

A collection of electrons will only act as a bunch when these electronsare introduced into a localized region in the radiation chamber beforethe electric field is established. If a collection of electrons aremoved into such a region after the electric field is established, thefixed phase relationship of the electrons will be destroyed; theelectrons will be randomly phased and, in so far as the generation ofenergy is concerned, will act as individual particles rather than as abunch.

Illustrative embodiments of my invention will now be described withreference to the accompanying drawings wherein:

Fig. 1 illustrates, in simplified form, an embodiment of my invention;

Fig. 2 is a graph of certain parameters of the embodiment of Fig. 1;

Fig. 3 is a graph of the cycloidal path of an electron bunch under theinfluence of mutually orthagonal magnetic and rotating electric fields;

Fig. 4 illustrates, in simplified form, apparatus for producing thecycloidal path shown in Fig. 3;

- Fig. 5 shows apparatus for compressing an electron bunch along a givenaxis;

Fig. 6 is an enlarged cross sectional view of an electron bunchcompressed along all radial directions perpendicular to the given axisof Fig. 5;

Figs. 7a and 7b are top and cross sectional views of a radiation chamberutilizing the apparatus of Figs. 5 and 6;

Fig. 8 illustrates an alternative form of the chamber shown in Figs. 7aand 7b; and

Fig. 9 shows a tube incorporating the radiation chamber of Figs. 7a and7b or Fig. 8.

Referring now to Fig. 1, a direct voltage V is applied with indicatedpolarity across terminals 50. One of these terminals is connected tometal plate 56; the other is connected through switch 52 to metal plate54. The volume subtended between the two plates is evacuated. Whenswitch 52 is closed, a constant electric field (neglecting fringeeffects) is established between the two plates, the electric fieldvector V having the direction indicated.

By means (not shown) a constant magnetic induction field is establishedbetween the plates in a direction perpendicular to the electric field(i.e. the magnetic field vector B points perpendicularly downward intothe plane of the paper as indicated on Fig. 1).

With switch 52 open, an electron is positioned at rest at point P withinthe evacuated volume subtended between the metal plates. As will becomemore apparent hereinafter, this evacuated volume constitutes a radiationchamber.

Since a magnetic field exerts no force on a stationary electron andsince no electric field is present, the electrons remain at rest.

Switch 52 is then closed and the electric field is established. Theelectric field exerts an electric force on the electron in such adirection that the electron travels toward the positively charged plate.As the electron moves, the magnetic field exerts a magnetic force on theelectron which is perpendicular both to the magnetic field vector and tothe instantaneous direction of electron movement. As a result of theinfluence of both of these forces, the electron travels toward the rightside of Fig. 1 in a cycloidal path 62 having cusps P P P,,.

The magnetic force is always exerted at right angles to theinstantaneous direction of electron motion; consequently, no energy canbe transferred between the magnetic field and the electron.

During its first half sector of travel along any are of path 62, energyis transferred to the electron from the electric field; during itssecond half of the sector of travel, energy is transferred from theelectron to the electric field. If the electron were not to lose energythrough radiation during the traversal of each semicircle, the energytransfer in the first half sector would equal the energy of transfer inthe second half sector, and the cusps of the path 62 would fall alongthe dotted line 58 which is parallel to both metal plates andperpendicular both to the electric field vector E and the magnetic fieldvector B However, since the electron is being constantly accelerated, itis constantly radiating energy; this radiated energy is supplied by theelectric field. Hence, the energy transferred from the field to theelectron during the first half sector of travel exceeds the energytransferred from the electron to the field during the second half sectorof travel by an amount equal to the radiated energy. As a result, thecusps of path 62 do not fall along the dotted line 58 but rather fallalong the solid line 60. Line 60 intersects line 58 at an acuate anglea.

Ultimately the electron is collected at plate 54. During the period inwhich the electron travels between the plates under the influence ofboth fields, the motion of the electron induces an increasing imagecharge on plate 54 and thus produces a displacement current flowingbetween the plates as shown in Fig. 2 where t defines the time at whichswitch 52 is closed and t, defines the time at which the charge iscollected.

Due to the acceleration of the electron, it radiates energy in the formof an electromagnetic wave. The wave-length of the radiated wave isinversely proportional to the magnetic field intensity. The tangent ofangle oz (the angle between lines 60 and 58 or stated differently, theangle between line 60 and a line perpendicular to both the electric andmagnetic field vectors) is directly proportional to the magnetic fieldintensity and hence is also inversely proportional to the wavelength ofthe radiated wave. The power radiated is proportional to the square ofthe electric field intensity (which in a typical example can be on theorder of '10 lO' volts per meter). Hence the wavelength of the radiatedwave decreases as the magnetic field intensity increases, while theradiated power increases as the electric field intensity increases.

When an electron hunch is used rather than a single electron (providedthat the bunch is positioned before the electric field is produced), thetotal power radiated by the bunch is no longer merely proportional tothe square of the electric field intensity, but rather is proportionalto the product of the square of this field intensity and the square ofthe number of electrons in the bunch. Hence the total power radiated canbe sharply increased by increasing the number of electrons in the bunch.In a typical example, the bunch will contain 10 -10 electrons.

It will be apparent that the device of Fig. 1 operates periodicallyrather than continuously, since subsequent to electron collection at thepositive plate the electric field must be cut off and a new bunch ofelectrons formed, before the electric field can be reestablished.

However, if the electric field is continuously rotated at a frequencymuch lower than the frequency of the cycloidal variation in a planeperpendicular to the magnetic field, continuous operation can beobtained. Fig. 3 illustrates this situation. The electric field vectorrotates in a circle; as a consequence, the cusps of the cycloidal bunchpath fall along a circle, i.e. the bunch describes an epicycloid. Thus,the electron bunch is not collected at a plate, but rather travelscontinuously along an endless closed path.

As indicated in Fig. 4, the field can be rotated by replacing the plates54 and 56 of Fig. 1 by two pairs of plates 64 and 66. The volumesubtended between these pairs of plates is evacuated. A first voltage Vis applied through transformer 68 between plates 64 and a second voltageV, is applied through transformer 70 between plates 66. Voltages V and Vare alternating voltages having the same frequency and amplitude butdisplaced in phase by relative to each other. This apparatus, as is wellknown to the art, establishes a uniformly rotating electric field withinthe evacuated volume subtended between the pairs of plates (theradiation chamber).

The device of Fig. 4 generates submillimeter waves in the same manner asthe device of Fig. 1, except that the operation is continuous ratherthan periodic.

Since the frequency of the electric field rotation is much lower thanthe frequency of the cycloidal motion, by considering the travel of theelectron bunch within a time interval sufiiciently short to permit theelectric field to be treated as if it were a constant rather than arotating field, it will be found that the same wavelength and powerconsiderations apply both to the device of Fig. 1 and the device of Fig.4. In Fig. 3, it will be seen that when the electric field is treated asa constant as for example represented by the solid vector E the tangentdrawn through cusp P is represented by the solid line 60 and intersectsline 58 (which is perpendicular both to the electric and magnetic fieldvectors) at angle a; this angle is identical with angle at of Fig. 1.

I have found it advantageous to cause an electron bunch to rotate in themanner shown in Fig. 3 and at the same time to periodically actuate anddeactuate the rotating electric field of Fig. 4 in the manner shown inFig. l.

A device of this type is shown in Figs. 7a and 7b. In order tounderstand this device, it will be first necessary to discuss theportion of the apparatus which produces the electric field as shown inFig. 5.

In Fig. 5, there is provided a radiation chamber which comprises ahollow cylinder into which are inserted upper and lower discs 102 and104. Disc 102 has a central orifice 105; disc 104 has no orifice. Forreasons which will become apparent hereinafter, disc 102 is designatedas a gate electrode while disc 104 is designated as a repellerelectrode. Cylinder 100 is connected to a point of high positivepotential V, and to terminal 112 of electronic switch 108. Repellerelectrode 104 is connected to a point of negative potential V; and isalso connected to terminal 106 of switch 108. Gate electrode 102 isconnected to terminal of switch 108. When switch 108 is in the positionindicated (for purposes of clarity, this switch is shown in block form,electronic switches being well known to the art), the gate electrode isat potential V,,. When the switch position is reversed, as for exampleunder the action of control pulses supplied to terminal 111, of switch108, the gate electrode is connected to the negative potential point V Aconventional pulsed cathode 114 generates an electron stream during eachinterval that switch 108 connects the gate electrode to the positivepotential point V,. This stream enters the radiation chamber through theaperture 105 of the gate electrode. Due to the negative potential V ofthe repeller electrode, the electrons in the stream are slowed down inthe direction of travel.

When the switch position is reversed, the cathode 114 is cut off, andthe negative potential on the two electrodes acts together with thepositive potential on the cylinder 100' to trap the electrons in thechamber and compress these electrons in direction parallel to the axisof the hollow cylinder 100.

However, the electrons are free to move radially outward from this axis.Therefore, it is necessary to prevent such movement by compressing thetrapped electrons along radial directions perpendicular to the axis ofcylinder 100, as shown in Fig. 6. This type of compression is obtainedthrough the action of a magnetic field produced by a coil 120concentrically mounted about the cylinder 100. As an electron beamenters the radiation chamber, the magnetic field compresses the beam inthe manner indicated.

A cross sectional detail view of this portion of the beam is shown inFig. 6. It will be seen that there is a particular potential variationbetween the beam and the cylinder 100. The potential profile of thisvariation is generated by rotating curve 122 about the axis of the beam,or stated difierently, curve 122 shows the potential profile in crosssection. In the region between the cylinder wall and the beam periphery,curve 122 describes a logarithmic function; in the region between thebeam periphery and the beam axis, curve 122 describes a parabolicfunction. When, as is required in my invention, the electric andmagnetic fields are sufficiently intense to prevent expansion of theelectron beam, this potential difierence is extremely high, for example,on the order of several thousands of volts; and the magnetic fieldintensity can be on the order of 10 -40 gauss.

The device of Figs. 7a and 7b incorporates the apparatus of Figs. 5 and6. However, it will be apparent that the electric field established asin Fig. 6 is used to compress the beam; it is parallel to the magneticfield and hence does not effect the cycloidal path shown in Fig. 3.

The electric field required to produce the cycloidal path is establishedby the use of two pairs of deflection electrodes 130, 132 and 134, 136which are parallel to the axis of the cylinder 100. These electrodes130, 132, 134 and 136 are equidistantly spaced circumferentially aboutthe cylinder axis. Electrodes 130 and 132 are equivalent to thedeflection plates 64 of Fig. 4, while electrodes 134 and 136 areequivalent to the deflection plates 66 of Fig. 4. The operations of theplates and electrodes are substantially identical. Note that in Figs. 7aand 7b the repeller electrode has the shape of an open cone with anorifice 140 to permit the radiated submillimeter waves to travel axiallydownward and out of the radiation chamber into space. At this point, thewaves can be utilized as required. Alternatively, the repeller electrodecan be a disc as before, but in this case can have a centralorificecontaining a window transparent to the submillimeter radiation.

' Fig. 8 shows a variation of the device shown in Figs. 7a and 7b,wherein the cylinder and deflection electrodes are structurallycombined, all other portions of these two devices being identical. InFig. 8 the cylinder is divided into four segments 150, 151, 152 and 153.A direct voltage V, is applied to all segments and establishes anaverage potential corresponding to that of the cylinder 100 of Fig. 8.Appropriate alternating voltages are applied to the segments in the samemanner as in Fig. 4, segments 150 and 153 being connected as plates 64of Fig. 4 and segments 151 and 153 being connected as 6 plates 66 ofFig. 4. The alternating voltages establish the rotating electric fieldused to produce the required cycloidal path.

Fig. 9 shows a tube 200 incorporating the device of Figs. 7a and 7b orFig. 8.

This tube has an evacuated envelope 204 in which is vertically mountedan electron gun 202 of the type suitable for pulsed operation as forexample the Pierce gun used in certain microwave tubes. The radiationchamber of Figs. 7a and 7b or Fig. 8 is mounted within the envelopebelow the gun. The repeller electrode of the chamber has a windowtransparent to the passage of the submillimeter waves. A conventionalhorn radiator 206 is attached to the envelope 204 to guide the radiatedwaves. A magnet coil 123 is concentrically mounted about the tubeenvelope in the region of the radiation chamber.

While I have shown and pointed out my invention as applied above, itwill be apparent to those skilled in the art that many modifications canbe made within the scope and sphere of my invention as defined in theclaims which follow. 1

What is claimed is:

1. In a wave generator, an evacuated radiation chamber; means toperiodically inject electrons into said chamber during discretely spacedperiods of time; means to establish a uniform magnetic field within saidchamber, the magnetic field vector pointing in a given fixed directionwithin said chamber, said magnetic field acting upon said injectedelectrons to compress said injected electrons within said chamber alongall directions perpendicular to said given direction; and meanssynchronized with said injection means to periodically establish anelectric field within said chamber during the intervals between saidelectron injection periods, said electric field acting upon saidinjected electrons to compress said injected electrons within saidchamber along all directions parallel to said given direction.

2. In a wave generator, a cylindrical evacuated radiation chamber; firstmeans to periodically inject electrons into said chamber duringdiscretely spaced periods of time; second means to establish a magneticfield within said chamber which acts upon said injected electrons inradial directions with respect to the axis of said chamber to radiallycompress said injected electrons within said chamber; and third meanssynchronized with said injection means to periodically establish anelectric compression field within said chamber during the intervalsbetween said electron injection periods, said compression field actingupon said injected electrons in directions parallel to the axis of saidchamber to axially compresss said injected electrons within saidchamber.

3. In a Wave generator, a cylindrical evacuated radiation chamber; firstmeans to periodically inject electrons into said chamber duringdiscretely spaced periods of time; second means to establish a magneticfield within said chamber which acts upon said injected electrons inradial directions with respect to the axis of said chamber to radiallycom-press said injected electrons within said chamber; third meanssynchronized with said injection means to periodically establish anelectric compression field within said chamber during the intervalsbetween said electron injection periods, said compression field actingupon said injected electrons in directions parallel to the axis of saidchamber to axially compress said injected electrons within saidchamber,whereby said compressed electrons constitute an electron bunch; andfourth means to establish another electric, field within said chamberwhich rotates at a constant speed in a plane perpendicular to the axisof said chamber, said rotating electric field and said magnetic fieldacting upon said bunch to cause said bunch to traverse an, epicycloidalpath, said bunch during said traversal radiating energy in the form ofan electromagnetic wave.

4. In a wave generator, a cylindrical evacuated radiation chamber; firstmeans to periodically inject electrons into said chamber duringdiscretely spaced periods of time, said electrons being injected along apath substantially coincident with the axis of said chamber; secondmeans synchronized with said first means to establish an electric fieldwithin said chamber, said electric field during said injection periods,acting upon said injected electrons in a direction parallel to the axisof said chamber to retard said injected electrons along said path, saidelectric field, during the intervals between said injection periods,acting upon said injected electrons in directions parallel to the axisof said chamber to axially compress said injected electrons within saidchamber; and third means to establish a magnetic field within saidchamber which acts upon said injected electrons in radial directionswith respect to the axis of said chamber to radially compress saidinjected electrons within said chamber.

5. In a wave generator, an evacuated radiation chamber comprising anelectrically conductive hollow cylinder open at both ends, a firstelectrically conductive electrode positioned within said cylinderadjacent one of said ends and separated from the cylinder wall, saidfirst electrode having a centrally positioned aperture, and a secondelectrically conductive electrode positioned within said cylinderadjacent the other end and separated from the cylinder wall, saidcylinder being maintained at a first direct potential with respect to areference potential, said second electrode being maintained at a seconddirect potential less positive than said first potential with respect tosaid reference potential; and means to periodically apply a thirdpotential more positive than said first potential with respect to saidreference potential to said first electrode during discretely spacedperiods of time, said first electrode being maintained at said secondpotential during the intervals between said periods.

6. In a wave generator, an evacuated radiation chamber comprising anelectrically conductive hollow cylinder open at both ends, a firstelectrically conductive electrode positioned within said cylinderadjacent one of said ends and separated from the cylinder wall, saidfirst electrode having a centrally positioned aperture, and a secondelectrically conductive electrode positioned within said cylinderadjacent the other end and separated from the cylinder wall, saidcylinder being maintained at a first direct potential with respect to areference potential, said second electrode being maintained at a seconddirect potential less positive than said first potential with respect tosaid reference potential; means to periodically apply a thirdjpotentialmore positive than said first potential with respect to said referencepotential to said first electrode during discretely spaced periods oftime, said first electrode being maintained at said second potentialduring the intervals between said periods; means to periodically injectelectrons into said chamber through the aperture of said first electrodeduring said periods; and means to establish a magnetic field within saidchamber, the magnetic field vector pointing in a direction parallel tothe axis of said cylinder whereby said injected electrons are bunched.

7. In a wave generator, an electrically conductive hollow cylinder openat both ends; a first electrically conductive electrode positionedwithin said cylinder adjacent one of said ends and separated from thecylinder wall, said first electrode having a centrally positionedaperture; a second electrically conductive electrode positioned withinsaid cylinder adjacent the other end and separated from the cylinderwall; third, fourth, fifth and sixth electrical-1y conductive electrodespositioned within said cylinder and separated from the cylinder wall andsaid first and second electrodes, said third, fourth, fifth and sixthelectrodes being equidistantly spaced radially about the axis of saidcylinder and extending in directions parallel to each other and parallelto said axis; first means to establish a first electric field betweensaid cylinder and said first and second electrodes; second means toestablish a second electric field between each of said third, fourth,fifth and sixth electrodes, said second field uniformly rotating in aplane perpendicular to said axis; and third means to establish amagnetic field within said space having its magnetic field vectorpointing in a direction always perpendicular to said rotating field.

References Cited in the file of this patent UNITED STATES PATENTS2,122,495 Scott July 5, 1938 2,158,114 Fritz May 16, 1939 2,233,779Fritz Mar. 4, 1941 2,270,777 Von Baeyer Jan. 20, 1942 2,414,121 PierceJan. 14, 1947 2,424,965 Brillouin Aug. 5, 1947 2,598,301 Rajchman May27, 1952 2,680,823 Dohler et al June 8, 1954 2,745,039 Bowen May 8, 19562,808,470 Hansell Oct. 1, 1957 2,824,997 Haeff Feb. 25, 1958

